The fabrication of semiconductor products, such as integrated circuits, often involves the formation of layers on a substrate, such as a silicon wafer. Various techniques have been developed for the deposition processes, as the layers often involve different materials. For example, a metal layer might be deposited and patterned to form conductive interconnects, or a dielectric layer might be formed to electrically insulate one conductive layer from another. Types of layer formation processes that have been used to form layers of dielectric materials and other materials include chemical vapor deposition (CVD) processes.
CVD processes include thermal deposition processes, in which precursor gases or vapors react in response to the heated surface of the substrate, as well as plasma-enhanced CVD (“PECVD”) processes, in which electromagnetic energy is applied to at least one precursor gas or vapor to transform the precursor into a more reactive plasma. Forming a plasma can lower the temperature required to form a film, increase the rate of formation, or both. Therefore, plasma-enhanced process are desirable in many applications.
When a layer is formed on a substrate, some material is usually also deposited as residue on exposed surfaces of the deposition chamber, including the chamber walls and gas distribution face plate. This material residue is generally undesirable because it can build up and become a source of particulate contamination, causing wafers to be rejected. Several cleaning procedures have been developed to remove residue from inside the chamber. One type of procedure, known as a “wet-clean” is performed by partially disassembling the deposition chamber and wiping the surfaces down with appropriate cleaning fluids. Other types of cleaning processes utilize a plasma to remove the residue by converting it to a volatile product that can be removed by the chamber exhaust system. These processes are known as “dry” cleans.
There are two general types of plasma dry cleaning processes. One type forms a plasma inside the processing chamber, or “in situ”. An example of an in situ plasma clean uses NF3 and C2F6 gases to form free fluorine for removing residue in the chamber interior.
Another approach to cleaning is to form a plasma in a remote plasma generator and then flow the ions into the processing chamber, typically in the same direction through the same components utilized to flow processing gases into the chamber. Such a remote plasma cleaning process offers several advantages, such as providing a dry clean capability to a deposition system that does not have an in situ plasma system. Furthermore, a remote plasma system may be more efficient at converting cleaning plasma precursor gases or vapors into a plasma, and forming the plasma outside the chamber protects the interior of the chamber from potentially undesirable by-products of the plasma formation process, such as plasma heating and sputtering effects.
There are, however, some less advantageous aspects associated with the utilization of remote plasmas for chamber cleaning. One issue is that the reactive ions in the remotely generated plasma may recombine to form less reactive molecular species as the plasma is transferred from the point of its generation to the processing chamber. Such unwanted recombination reduces the effective concentration of the reactive ions in the chamber, prolonging cleaning times and elevating cost due to the increased consumption of expensive, environmentally-friendly cleaning gases such as NF3.
Typically, the remotely generated plasma experiences a pressure increase as it is flowed through constrictions, as are presented by blocker plate and shower head components responsible for precise distribution of processing gases during deposition or etching. In the context of flowing remotely generated plasmas into the chamber for cleaning, these pressure increases cause unwanted recombination into relatively unreactive molecular species, diluting the effective concentration of reactive ions immediately downstream of the shower head and blocker plate. However, it is the surfaces immediately downstream of the blocker plate and shower head which typically experience the greatest formation of residue and hence require the most cleaning.
Therefore, there is a need in the art for methods and apparatuses which reduce the recombination of ions in a remotely-generated plasma flowed into a processing chamber, thereby maximizing the cleaning effect of the remote plasma.